1. Field of Invention
This invention relates to a system, including a method and apparatus using heat conduction as a means to control temperatures in the growth of plants of various kinds. The system has application for improving the growth of all horticulturual products affected by temperature variations, such as plants, flowers, crops, vegetable produce and fruits, and is especially useful in conjucntion with hydroponic plant growing systems in which the plants are grown in a nutrient solution with a growing medium to support the roots.
2. Description of the Prior Art
It is well known in the art of plant growing that growth is highly dependent on such factors as light, temperature, atmospheric gases and humidity. With particular reference to temperature, it is well known that plant growth can be increased or maximized by maintaining optimal plant growing temperatures. See, e.g., Pierce, Green House Grow How, (Lakeside Press 1977) at 27. Accordingly, the most direct and accurate temperature controls will maximize plant growth.
Prior art systems for controlling growing temperatures have primarily utilized enclosures, such as greenhouses, in which the overall ambient air temperature is controlled, usually by varying radiant heat from solar or artificial sources, use of heat pipes in contact with the air, or air heat pumps or air conditioners, or various combinations thereof. Such systems are disclosed, for example, in U.S. Pat. Nos. 1,212,097, 2,897,631, 2,153,871, 3,935,673, 4,163,342, 4,198,783, 4,292,762.
In these prior art systems, the temperature of the actual growing system of the plants, including the soil, roots, and plant structure, will in time move toward equilibrium with the temperature of the ambient air, the heat transfer method being primarily by convection.
Some systems have also used a partially conductive heat transfer, as in U.S. Pat. No. 4,320,594 for a Mass Algal Culture System, which is designed to affect the conditions for algae growth in a continuously moving liquid medium. U.S. Pat. No. 4,486,977 discloses a temperature controlled liquid flowing in exposed passages providing contact with surrounding air and parts of the structures for growing plants.
As is well known, air is less efficient for the transfer of heat energy than more dense substances, such as metal. Indeed air is used as an insulator such as found in double-walled window panes and what is known as a "double poly" greenhouse.
In addition, temperature gradients in a volume of air will limit the ability to control temperatures of articles in contact with such air.
Further, the convection and radiant heat transfer methods of the prior art may require considerably more time than a conductive heat transfer method to bring the overall temperature of a growing system to steady state conditions, or to correct variations which occur due to factors outside the enclosed growing environment.
The prior art systems using primarily temperature control of air, or heat transfer by convection of air suffer from temperature variations and require an unnecessarily lengthened period of time to achieve a steady state temperature in the plant growing system. In addition, control of air temperatures in an enclosed volume of air does not provide optimal temperature control within the growing medium itself. Further, the heating and cooling of large volumes of air is generally less efficient and more costly as a heat transfer method than controlling a confined and smaller volume of water or other liquid, having a higher heat conductivity than air.
U.S. Pat. No. 4,309,843 to Kato discloses a plant cultivation device using hot water pipes covered by "a layer of wood chips" over which is placed the "cultivation soil" Col. 1, lines 46-50. While Kato thus utilizes some conductive heat transfer, the layer of wood chips underneath the soil tends to impede the heat transfer from the liquid in the pipes, in a manner opposite the method and structure claimed in the present invention. Kato suffers from the additional drawbacks that its heating pipes are enclosed in a complex structure, as disclosed and claimed by Kato, involving concrete blocks 5, slate-corrugated plate 6, partition wall plates 2 and shielding cover 11. Being in effect buried under the growing medium, the pipes of Kato are not exposed, cannot be readily maintained, and will inhibit early detection of failures, such as leaks causing flooding of the growing medium and destruction of the plants.
Even temperature control systems adding open passages of or other contact with a temperature controlled fluid do not disclose the superior temperature control methods and apparatus of the present invention.